Microscopic examination of unstained cell and tissue preparations often suffers from a lack of contrast between individual cells and the background matrix or between individual parts of cells. In order to alleviate this difficulty, dyes (stains) that are taken up differentially by cells or parts of cells have been used in microscopic examination of tissues for over a century.
Because of the manner in which slides with tissue preparations are prepared (see Elias, J., "Immunohistopathology: A practical Approach to Diagnosis" ASCO Press, 1990, pp. 3-4, for examples of such preparation), the size and/or location of a tissue sample on a microscope slide can vary considerably within a relatively large area of the slide. In order to apply a stain to the correct location on a slide and to provide rinsing and other manipulation steps at appropriate times and in fight amounts, until recently all such staining operations were carried out by hand. It is apparent, however, that modem laboratories that examine large numbers of tissue preparations find it desirable to automate the staining process. Accordingly, a number of manufacturers have developed equipment for automated staining of tissue preparations on microscope slides.
For example, U.S. Pat. No. 4,985,206 describes an apparatus and process for automating the application of staining reagents to a thin tissue section mounted on a microscope slide. The apparatus and method use a channel-defining element that is assembled with the microscope slide to provide an enclosure of capillary dimensions into which liquids can be injected. Liquids are added sequentially to the capillary space, where the addition of a new liquid forces out the previous liquid. A plurality of these assemblies of microscope slides and specialized covers can be placed in a rack on an apparatus for automated addition of liquids.
A further automated immuno staining apparatus referred to as the Ventana 320.TM. is produced by Ventana Medical Systems, Inc. of Tucson, Ariz. This apparatus applies a liquid known as Liquid Coverslip.TM. to each slide prior to reagent addition. Liquid Coverslip.TM. is a non-aqueous material having a density less than that of water. When a reagent dissolved in water is added to a microscope slide, the reagent sinks to be bottom of the Liquid Coverslip.TM. for spreading across the surface of the slide. Slides are organized on a carousel which rotates beneath a dispensing head for application of reagents or wash fluids.
Leica produces an automated staining apparatus known as the Jung Histostainer Ig.TM.. This is another carousel-type device, but reagents are applied by a spraying operation rather than by dropping liquid onto an organic film. The apparatus contains a permanent reagent spraying head that can be moved along a single axis to provide spray coverage over a microscope slide located on the rotating tray when the slide is rotated into a position underneath the head. Excess reagent is removed by a permanent clearing nozzle which blows air in a pressure front across the slide, forcing excess liquid off at the completion of the reagent incubation step.
All of these apparatuses have attempted to solve certain conflicting goals in automated apparatuses of this type. For example, it is desirable to use a minimum of expensive or toxic reagents, particularly reagents used in immuno staining (e.g., antibodies and other reagents of biological origin), while insuring that complete coverage of the microscope slide by the reagent will occur. However, the last-referenced spraying operation above uses an excess of reagent which must be removed in order to obtain satisfactory coverage. The other techniques require additional manipulative steps, such as the use of a hand-assembled cover or an additional liquid reagent to provide for proper spreading of the reagent. It would be desirable to have an automated apparatus that can use regular microscope slides without additional manipulation and that does not require the use of either excess reagent or the use of an organic liquid with additional manipulation and disposal steps.
Accordingly, further developments that allow individual slides to be treated differently in a single batch operation and that provide an automated procedure that uses reagents efficiently remain needed.